266
D. H. CUSHINO
The dynamic boundary at about 100 km from the shore marks the
edge of the coastal upwelling, as noted in the previous section. But
divergences continue to seaward and the biological boundary must be
at the imprecise edge of the wind system. The boundary in zooplankton
(Thrailkill, 1956, 1957, 1959, 1961, 1963) or surface phosphate-phosphorus (California, Department of Fish and Game, 1958) is often beyond
the dynamic boundary, perhaps hundreds of kilometres. As a convention, the biological limit of the upwelling area is taken as half the
maximum from the coast. Zooplankton quantity is used as a convenient
index of summed production over a period and phosphate-phosphorus
is used because it correlates so well with the zooplankton; this remarkable correlation is discussed in detail in the following section.
Some evidence will be put forward below to show that the width of the
zone of biological production as defined above is about two and a half
times the distance to the dynamic boundary.
The production cycle in the upwelling areas is noticeable because it
is really a discontinuous temperate cycle of high amplitude which is
contrasted with a continuous sub-tropical cycle of low amplitude in the
deep ocean well offshore of the upwelling area. Then it is not accidental
that the whole community structure as shown in the California current
is really a temperate one as will be shown below. The plankton communities are much less diverse than those of the deep tropical ocean ;
the bathypelagic fishes of the deep ocean are not very abundant in the
upwelling zones proper, but like the tunas they are found outside the
dynamic boundary 100 km off the coast. Thus, an upwelling area outside the subtropical anticyclones may be of relatively little importance
because the effect of upwelling cannot be distinguished from the
ordinary temperate cycle of production. In the Antarctic there is a
slow and extensive high amplitude pulse of production which is correlated with the seasonal changes of light intensity ; the existence of
deep upwelling at all seasons against the shelf is an unnecessary source
of explanation.
Thus, for physical and biologicd reasons, the study of upwelling is
effectively limited to the subtropical anticyclones. The cycle of produchion in an upwelling area is considered as a temperate cycle rising
slowly from the bottom of a fairly deep photic layer. Because of a slow
and continuous mixture, with a steady addition of living material at
the coast and offshore, the distribution of biomass in an upwelling area
is fairly uniform. Because divergences continue far offshore beyond the
dynamic boundary at 100 km, the width of the upwelling area, biologically, is considered to extend beyond the obvious physical boundaries.
D. H. CUSHINO
The dynamic boundary at about 100 km from the shore marks the
edge of the coastal upwelling, as noted in the previous section. But
divergences continue to seaward and the biological boundary must be
at the imprecise edge of the wind system. The boundary in zooplankton
(Thrailkill, 1956, 1957, 1959, 1961, 1963) or surface phosphate-phosphorus (California, Department of Fish and Game, 1958) is often beyond
the dynamic boundary, perhaps hundreds of kilometres. As a convention, the biological limit of the upwelling area is taken as half the
maximum from the coast. Zooplankton quantity is used as a convenient
index of summed production over a period and phosphate-phosphorus
is used because it correlates so well with the zooplankton; this remarkable correlation is discussed in detail in the following section.
Some evidence will be put forward below to show that the width of the
zone of biological production as defined above is about two and a half
times the distance to the dynamic boundary.
The production cycle in the upwelling areas is noticeable because it
is really a discontinuous temperate cycle of high amplitude which is
contrasted with a continuous sub-tropical cycle of low amplitude in the
deep ocean well offshore of the upwelling area. Then it is not accidental
that the whole community structure as shown in the California current
is really a temperate one as will be shown below. The plankton communities are much less diverse than those of the deep tropical ocean ;
the bathypelagic fishes of the deep ocean are not very abundant in the
upwelling zones proper, but like the tunas they are found outside the
dynamic boundary 100 km off the coast. Thus, an upwelling area outside the subtropical anticyclones may be of relatively little importance
because the effect of upwelling cannot be distinguished from the
ordinary temperate cycle of production. In the Antarctic there is a
slow and extensive high amplitude pulse of production which is correlated with the seasonal changes of light intensity ; the existence of
deep upwelling at all seasons against the shelf is an unnecessary source
of explanation.
Thus, for physical and biologicd reasons, the study of upwelling is
effectively limited to the subtropical anticyclones. The cycle of produchion in an upwelling area is considered as a temperate cycle rising
slowly from the bottom of a fairly deep photic layer. Because of a slow
and continuous mixture, with a steady addition of living material at
the coast and offshore, the distribution of biomass in an upwelling area
is fairly uniform. Because divergences continue far offshore beyond the
dynamic boundary at 100 km, the width of the upwelling area, biologically, is considered to extend beyond the obvious physical boundaries.